Spin state crossover in Co3BO5 N.V. Kazak, M.S. Platunov, Yu.V. Knyazev, M.S. Molokeev, M.V. Gorev, S.G. Ovchinnikov Kirensky Institute of Physics, Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia Z.V. Pchelkina1,2, V.V. Gapontsev2, S.V. Streltsov1,2 1 M.N. Miheev Institute of Metal Physics UB RAS, 620137 Ekaterinburg, Russia 2Ural Federal University, 620002 Ekaterinburg, Russia J. Bartolomé3, A. Arauzo3,4 3Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza and Departamento de Física de la Materia Condensada, 50009 Zaragoza, Spain 4Servicio de Medidas Físicas, Universidad de Zaragoza, Zaragoza, Spain V.V. Yumashev Institute of Chemistry and Chemical Technology, Federal Research Center KSC SB RAS, 660036, Krasnoyarsk, Russia S.Yu. Gavrilkin P.N. Lebedev Physical Institute of RAS, 119991 Moscow, Russia F. Wilhelm, A. Rogalev ESRF-The European Synchrotron, 71 Avenue des Martyrs CS40220, F-38043 Grenoble Cedex 9, France 3+ Abstract We have investigated the magnetic contribution of the Co ions in Co3BO5 using the X-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. The XMCD experiments have been performed at Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds providing strong experimental support for the low-spin Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100-250 K correlates well with two Co2+ ions in the high-spin state and some orbital contribution. The crystal structure of Co3BO5 single crystal has been solved in detail at the T range 296-703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment and oxidation state of Co4 site strongly change with heating. The GGA+U calculations have revealed that at low-temperatures the system is insulating with the band gap of 1.4 eV and the Co2+ ions are in the high-spin state, while Co3+ are in the low-spin state. At high temperatures (T>700 K) the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and high-spin state. PACS number(s): 75.50.Gg, 75.30.Wx, 75.30.Gw 1. INTRODUCTION The cobalt oxides belong to a large class of strongly correlated compounds showing the complex interplay between spin, charge, lattice, and orbital degrees of freedom. Like other 3d transition metals cobalt exhibits several possible oxidation states – Co2+(d7), Co3+(d6) and Co4+(d5). A property which makes the cobalt oxides very peculiar is the ability of Co3+ ions to accommodate various spin states, that is, low spin (LS), high spin (HS) and intermediate spin 1 (IS). The spin state of Co3+ appears to be very sensitive to changes in the Co-O bond length and Co-O-Co bond angle and a spin state transition could happens under action of temperature, magnetic field or external pressure. This makes the physics of the cobalt oxides very complicated being a subject of a long-standing controversy [1]. Recently, cobalt-containing oxyborates with a 2+ 3+ general formula of M 2Mʹ BO5 (M, Mʹ = Co, and 3d metal ions, as well as Al, Ga, and Mg) have attracted much attention due to the discovery of their intriguing magnetic and electronic behaviors [2-7]. These materials crystallize in orthorhombic structure (Sp.gr. Pbam) and are isostructural to ludwigite mineral. The M2+ and Mʹ3+ ions are located at the centers of edge sharing oxygen octahedra forming linear chains propagating along the short crystallographic direction (c≈3 Å). The boron atoms have a trigonal-planar coordination BO3, normally linked via common corners with oxygen octahedra. The cations occupy four crystallographically distinct metal sites 2a, 2b, 4g, and 4h, which are usually numbered as M1, M2, M3, and M4, respectively (Fig. 1). The first three are occupied by divalent metal ions, whereas the latter site located in the spaces between BO3-groups is occupied by the trivalent cations. As a result, a dense crystal structure is formed by alternating layers of divalent and trivalent cations. The triads 3-1-3 and 4- 2-4 with longest and shortest interionic distances are structurally, magnetically and electronically singled out. The common ludwigite structure allows various types of magnetic interactions involving the metal ions, including superexchange interactions, direct exchange interactions, and dipole-dipole interactions, among which the former is dominant. a) Co2+ 4 Co3+/B3+ 3 1 3 2 4 b) c) 3 1 3 4 2 4 ~3.31 Å ~2.75 Å 2 Fig. 1.a) The ab projection of crystal structure of Co3BO5. The green and blue spheres are octahedrally coordinated divalent and trivalent ions, respectively, that occupy different crystallographic sites 1, 2, 3, and 4. The triangle BO3- groups are shown by yellow. b) The triads 3-1-3 with the longest (b) and 4-2-4 with shortest Co-Co distances (c) interionic distances are highlighted by dotted lines The homometallic ludwigites, Fe3BO5 and Co3BO5, are the most thoroughly studied systems due to their quite distinct structural, magnetic and electronic properties [2,8]. Fe3BO5 demonstrates extremely rich physics and undergoes several transitions upon cooling from room temperature. First, a structural orthorhombic – orthorhombic transition takes place at Tst = 283 K (Pbam(№55) – Pbnm(№62)), which is accompanied by the formation of the pairs (“dimers”) of 2+ 3+ Fe and Fe ions in 4-2-4 triad [9,10]. The phase transition in Fe3BO5 detected by the X-ray diffraction also manifests itself in the temperature dependence of electrical-resistivity and hyperfine parameters, revealing well-defined anomalies at Tst [9,11,12]. With decreasing temperature a cascade of magnetic transitions is observed by means of magnetometry, Mössbauer, and temperature-dependent neutron diffraction studies [13-15]. At TN1=112 K the 4- 2-4 spin ladder is antiferromagnetically ordered. At TN2=74 K the ferrimagnetic ordering in the 3-1-3 spin-ladder appears. And finally, the transition to the antiferromagnetic ground state at TN3=30 K with zero magnetic moment per unit cell is observed. The bulk magnetization measurements revealed that the anisotropy axis changes from the a to the b axis in the low- temperature antiferromagnetic phase. On the contrary, cobalt ludwigite Co3BO5 demonstrates more conventional behavior, with the only ferrimagnetic transition at TN=42 K and no structural transformations [4, 16,17]. The high magnetic uniaxial anisotropy with the b-axis as an easy magnetization direction was detected in the entire temperature range [15,18]. The remanent magnetization per Co atom is ~ 1.1 μB. A small slope of magnetization at high magnetic fields seems to indicate the existence of a more complex magnetic structure. Over the last decade, the attempts to understand the observed difference in magnetic and electronic properties of two homometalic ludwigites led to a synthesis of several new compounds: CoMgGaBO5 [19], Co2.4Ga0.6BO5 [20], Co2.88Cu0.12BO5 [21], Co4.76Al1.24BO5 [22], Co2AlBO5 [23], Co3-xFexBO5 (0.0<x<1.0) [15,17,18,24,25], Co1.7Mn1.3BO5 [26], Co5TiB2O10 [27], and Co5SnB2O10 [28] are some of them. (Here we do not mention ludwigites based on other 3d metal, the studies of which are also numerous). The main conclusions that can be drawn from these studies are the following: i) isovalent substitution of Co3+ ions by nonmagnetic ions like Ga and Al or magnetic Mn causes an onset of short-range or long-range orderings, but their critical temperatures are found in the vicinity of TN characteristic of Co3BO5. The reason, apparently, is 3 the sensitivity of the magnetic subsystem to the cation distribution. ii) The nonmagnetic substitution of type 2·Co3+→(Co2++M4+), where M4+=Ti or Sn leads to an antiferromagnetic spin ordering at TN=82 K, i.e. twice of TN of Co3BO5. Taking into account that tetravalent substitution generates Co2+ ions at the M4 site, an increase in the Neel temperature should be attributed to the enhancement of exchange interactions via this site. iii) An unexpected effect was discovered at the substitution of Co3+ by Fe3+ ions. In particular, with Fe3+ content increase the samples exhibit a magnetic transition at 83 K, which is then split into two magnetic transitions at 77 and 86 K, 3+ and finally, the Fe -rich sample Co2FeBO5 shows antiferromagnetic (TN1=110 K) and ferrimagnetic-like (TN2=70 K) transitions similar to the end compound Fe3BO5. An analysis of the Co-containing ludwigite systems indicates that the chemical substitution modifies the magnetically active subsystem only if it results in the appearance of magnetic ions at M4 site (Co2+, Mn3+, or Fe3+). Based on the low-temperature neutron diffraction, Freitas et.al [29] have found a rather small magnetic moment (~0.5 μB) for Co ions occupying the M4 site, instead of the expected 4 3+ 4 2 μB for high-spin Co (t2g eg , S=2), while the observed moments of Co at the M1 site (~3.6 μB), at the M2 site (3.1 μB), and at the M3 (3.8 μB) are consistent with those expected for high-spin 2+ 5 2 3+ Co ions (t2g eg , S=3/2). It was then assumed that Co ions in Co3BO5 are in the low-spin state 6 0 (t2g eg , S=0). It was found also that all magnetic moments are almost parallel to the b-axis, making it an easy magnetization direction. The spin ordering within 3-1-3 triad is antiferromagnetic (↑↓↑). These triads are ferromagnetically coupled with each other via Co ions at the M2 site resulting in the total uncompensated magnetic moment ~1.4 μB per Co ion.